Powder injection molding composition

ABSTRACT

An powder injection molding composition is disclosed. The composition comprises caprolactam and a plurality of particles, where that plurality of particles is selected from a metal powder, a metal hydride powder, a ceramic powder, a ferrite powder, and mixtures thereof. The composition optionally further comprises a wax and polymeric material.

FIELD OF THE INVENTION

The invention relates to a molding composition which includes a metalpowder, a ceramic powder, and/or a ferrite powder, in combination with abinder component comprising one or more nitrogen-containing compounds.The invention further relates to a method to form a shaped article usingApplicant's molding composition.

BACKGROUND OF THE INVENTION

Powder Injection Molding (PIM) is an attractive process to form shapedparts. Using such a PIM process, metal, ceramic, and/or ferrite powderis combined with a carrier, such as one or more carbon-containingmaterials, and that resulting mixture is molded into a desired shape,sometimes called a “green body.” The one or more carbon-containingmaterials functions as, among other things, a binder which facilitatesmolding of the metal/ceramic/ferrite powder into the green body. The oneor more carbon-containing materials are then removed via thermal and/orsolvent means. Thereafter, the remaining shaped powder is sintered toproduce the desired metal/ceramic/ferrite shaped article. In order tominimize, and hopefully to, prevent the formation of defects in, and/orinclusion of impurities in, the final shaped article, it is desirablethat the one or more carbon-containing materials be completely removedfrom the green body prior to sintering.

PIM is a particularly appealing process for forming shaped itemscomprising titanium due to: low processing temperatures, longer moldlifetimes, and ability to produce near net shaped parts requiringminimal final machining. As those skilled in the art will appreciate,shaped titanium parts have utility as medical implants, i.e. bone screwsand plates, golf club heads, and as aerospace components. Despite theseadvantages, however, the high reactivity of titanium and itssusceptibility towards forming solid solutions with commonly occurringelements (i.e. oxygen, carbon, and nitrogen), requires that the one ormore carbon-containing materials be completely removed from the shapedbody at temperatures below about 500° C., and more preferably near 450°C. Applicant's invention comprises a PIM formulation and method whichproduces a low viscosity, easily moldable composition to form greentitanium body which can be debound and sintered to produce, for example,titanium parts having a bulk density almost equal to the density ofnaturally-occurring titanium metal.

SUMMARY OF THE INVENTION

Applicant's invention includes a molding composition, comprising aplurality of particles, where those particles are selected from a metalpowder, a ceramic powder, a ferrite powder, and mixtures thereof, incombination with one more nitrogen-containing compounds comprisingbetween about 3 and about 20 carbon atoms. Applicant's invention furtherincludes a method to form a shaped article. Applicant's method firstprovides one or more nitrogen-containing compounds comprising betweenabout 3 carbon atoms and about 20 carbon atoms. Those one or morenitrogen-containing compounds are then liquefied to form a molten bindercomponent.

Applicant's method then provides a plurality of particles, where thoseparticles are selected from the group consisting of a metal powder, aceramic powder, a ferrite powder, and combinations thereof, and forms amixture comprising the plurality of particles and the molten bindercomponent. That liquid particle/binder mixture is introduced into a moldhaving an internal cavity formed in a desired shape. The particle/bindermixture is allowed to solidify to form a shaped green body. That shapedgreen body is heated at up to 2500° C. to form a shaped article.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from a reading of the followingdetailed description taken in conjunction with the drawings in whichlike reference designators are used to designate like elements, and inwhich:

FIG. 1 is flow chart summarizing the initial steps of Applicant'smethod;

FIG. 2 is a flow chart summarizing additional steps in Applicant'smethod;

FIG. 3 is a flow chart summarizing the final steps in Applicant'smethod.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In certain embodiments, Applicant's invention includes a moldingcomposition comprising titanium powder, titanium hydride powder,zirconium powder, rhenium powder, tantalum powder, tungsten carbidepowder, ferrite powder, ceramic powder and mixtures thereof. By “ferritepowder,” Applicant means an iron-based ferromagnetic powder, includingBaFe₁₂O₁₉, SrFe₁₂O₁₉, and the like, and mixtures thereof. By “ceramicpowder,” Applicant means a powder formed from inorganic, nonmetallicmaterials which are typically crystalline in nature and comprisecompounds formed between metallic and nonmetallic elements, such asaluminum and oxygen (alumina-Al₂O₃), calcium and oxygen (calcia-CaO),and silicon and nitrogen (silicon nitride-Si₃N₄).

In certain of these embodiments, the metal/ceramic/ferrite powdercomprises particles having an average diameter between about 10 micronsand about 150 microns. In certain embodiments, Applicant's compositionincludes powder formed of particles having an average diameter of about45 microns. The particles can be either irregularly-shaped,spherically-shaped, or a combination of both.

In the titanium powder embodiments, the titanium powder is of a kindsometimes called “chemically pure (CP),” and/or ASTM Titanium Alloygrades 1 through 4. Such grades are well known to those skilled in theart of powder metallurgy.

In alternative embodiments, Applicant's invention includes a moldingcomposition comprising a powder formed of a titanium alloy of thegeneral formula Ti_(x)M1_(y)M2_(z), and wherein M1 & M2 are metalsselected from the group consisting of Aluminum, Vanadium, Molybdenum,Chromium, Tin, Zirconium, Manganese, Silicon, and Palladium Examples ofsuitable titanium-based alloys include but are not limited to thefollowing compositions:Ti-6Al-4VTi-5Al-2.5SnTi-5Al-6Sn-2Zr-1Mo-0.2SiTi-6Al-2Sn-4Zr-2MoTi-8Al-1Mo-1VTi-6Al-6V-2SnTi-6Al-2Sn-4Zr-6Mo,Ti-8MnTi-8Mo-8V-2Fe-3AlTi-13V-11Cr-3AlTi-11.5Mo-6Zr-4.5SnIn certain embodiments, Applicant's molding composition comprising atitanium alloy of the formula Ti-6Al-4V in powder form, wherein thatpowder comprises particles having the dimensions and shape(s) describedabove.

In alternative embodiments, Applicant's invention includes a moldingcomposition comprising a powder formed of a titanium and/or a titaniumalloy in combination with one or more ceramic particulate/fibrousreinforcing phase. Examples of such ceramic reinforcement phases includeone or more carbides, oxides, and nitrides of boron, silicon, aluminum,titanium, and mixtures thereof.

In certain embodiments of Applicant's invention, the titanium/titaniumalloy component is present in Applicant's molding composition at a levelbetween about 70 weight percent and about 90 weight percent. In certainembodiments of Applicant's invention, the titanium/titanium alloycomponent of Applicant's molding composition is present at a levelbetween about 75 weight percent and about 85 weight percent.

In certain embodiments, Applicant's invention includes a moldingcomposition comprising one more nitrogen-containing compounds comprisingbetween about 3 and about 20 carbon atoms. In certain embodiments, theone or more nitrogen-containing compounds have structure I, II, III, IV,V, VI, and/or VII:

wherein n is between about 1 and about 12, and where R1, R2, R3, R4, R5,R6, R7, R8, R9, R10, R11, R12, and R13, are each independently selectedfrom the group which includes hydrogen, alkyl, cycloalkyl, vinyl,alkenyl, cycloalkenyl, phenyl, and benzyl.

For example, ε-caprolactam, structure I where n=3 and R1 is H, melts at70° C. to give a low viscosity fluid (12.3 cP). Applicant has found thatthe caprolactam melt readily wets the surface of titanium particles,thereby facilitating relatively uniform dispersion of those particles inthe melt. In addition, caprolactam has low toxicity and is readilyavailable commercially in high purity. Furthermore, Applicant has foundthat caprolactam-based PIM formulations rapidly solidify upon cooling toform a green body having superior mechanical properties which facilitateeasily handling and subsequent processing.

In certain embodiments of Applicant's molding composition, the one ormore nitrogen-containing compounds having structure I, II, III, IV,and/or V, are present from about 1 weight percent to about 30 weightpercent. In other embodiments, those one or more nitrogen-containingcompounds are present from about 15 weight percent to about 25 weightpercent.

In certain embodiments of Applicant's invention, Applicant's compositionincludes one or more additives. These additives enhance the moldingcharacteristics of Applicant's composition, and enhance the strength ofthe green body formed using Applicant's composition and method. Incertain embodiments, these one or more additives are present inApplicant's composition in an aggregate amount of between about 0.1weight percent and about 5.0 weight percent. In certain embodiments,these one or more additives are present in an aggregate amount ofbetween about 1.0 weight percent and about 3.0 weight percent.

Certain embodiments of Applicant's composition include one or morepolymeric materials. Such polymeric materials include homopolymersand/or copolymers formed from ethylene, propylene, acrylic acid, acrylicacid esters, methacrylic acid, methacrylic acid esters, N-vinylpyridine, N-vinyl pyrrolidone, N-vinyl caprolactam, maleic anhydride andcombinations thereof. For example, ethylene-co-acrylic acid (EAA)polymers and copolymers have an affinity for titanium surfaces, andthermally decompose at temperatures below 450° C. In other embodiments,Applicant's composition includes polystyrene (PS). In yet otherembodiments, Applicant's composition includes poly-α-methylstyrene(PMS). Both PS and PMS thermally decompose and unzip at temperaturesbelow 370° C. By “unzip,” Applicant means a process wherein the polymerchain decomposes into individual monomer units.

The following Example I is presented to further illustrate to personsskilled in the art how to make and use the invention and to identifypresently preferred embodiments thereof. This Example I is not intended,however, as a imitation upon the scope of Applicant's invention, whichis defined only by the appended claims.

Example I

The formulation of Example I includes a metal component and a bindercomponent. Table I recites the binder component of this Example I.

TABLE I Weight Percent Of Component Non-Metal Portion Caprolactam 94.0A-C 5120 Ethylene-co-Acrylic Acid Polymer* 6.0 *Honeywell PerformancePolymers, Morristown, N.J. Acid #120 mg KOH/g.

Table II recites the complete formulation of this Example I.

TABLE II Weight Component Percent Of Formulation Titanium powder 81.7Caprolactam 17.2 A-C 5120 Ethylene-co-Acrylic Acid Polymer 1.1The product of EXAMPLE I, when processed using the steps of FIG. 1,discussed below, yields a green body having a 49.8 volume percent solidscontent.

The following Example II is presented to further illustrate to personsskilled in the art how to make and use the invention and to identifypresently preferred embodiments thereof. This Example II is notintended, however, as a limitation upon the scope of the invention,which is defined only by the appended claims.

Example II

The formulation of this Example II includes a metal portion and a binderportion. Table III recites the components comprising the binder portion.

TABLE III Weight Percent Component Of Non-Metal Portion Caprolactam 94.3A-C 5120 Ethylene-co-Acrylic Acid Polymer* 4.3 Primacor 5990IPolyethylene-co-Acrylic Acid** 1.4 *Honeywell Performance Polymers,Morristown, NJ **Dow Chemical Corporation, Midland, MI

Table IV recites the components, and the weight percentages of thosecomponents, in the formulation of this Example II.

TABLE IV Weight Percent Component Of Formulation Titanium powder 79Caprolactam 19.8 A-C 5120 Ethylene-co-Acrylic Acid Polymer 0.91 Primacor5990I Polyethylene-co-Acrylic Acid 0.29

The product of EXAMPLE II, when processed using the steps of FIG. 1,discussed below, yields a green body having a 45.7 volume percent solidscontent. In accord with step 130 (FIG. 1) of Applicant's methoddiscussed below, the A-C 5120 component and the Primacor component werecombined in the melt to form about a 84.6% A-C 5120/5.4% Primacor weightpercentage mixture. In accord with step 150 (FIG. 1), that additivecombination was then added to molten caprolactam.

The following Example III is presented to further illustrate to personsskilled in the art how to make and use the invention and to identifypresently preferred embodiments thereof. This Example III is notintended, however, as a limitation upon the scope of the invention,which is defined only by the appended claims.

Example III

The formulation of this Example III includes a metal portion and abinder portion. Table V recites the components comprising the binderportion.

TABLE V Weight Component Percent Of Non-Metal Portion Caprolactam 87.5Endex 160 12.5 Poly[styrene-co-α-methylstyrene]* *Eastman Chemical Corp.Kingsport, TN

Table VI recites the components, and the weight percentages of thosecomponents, in the formulation of this Example III.

TABLE VI Component Concentration (wt. %) Titanium powder 77.6Caprolactam 19.6 Endex 160 2.8 Polystyrene-co-α-methylstyrene*The product of EXAMPLE III, when processed using the steps of FIG. 1,discussed below, yields a green body having a 43.5 volume percent solidscontent.

Examples I, II, and III, include a single phase non-metallic portion. Incertain embodiments, Applicant's composition includes a two phasenon-metallic portion. The addition of wax to the caprolactam produces atwo phase organic binder which is readily moldable, solidifies rapidly,while allowing a significant portion of the binder can be removed viasolvent extraction prior to thermal debinding operations. By “wax”,Applicant means a material having a molecular weight of about 5,000Daltons or less, which is solid at room temperature. In certainembodiments, the wax component of Applicant's composition comprises oneor more esters, one or more fatty acids, one or more alcohols, one ormore hydrocarbons, and combinations thereof.

Solvent extractable binder systems have a significant advantage overtraditional thermal debinding systems. In particular, diffusion is therate limiting step for binder removal during thermal operations and thebinder diffusivity scales with green part thickness according to thesquare root of time,X²≈2Dtwhere X represents the semi-thickness for flat plate green specimen, Dcomprises the Diffusivity of one or more components from thenon-metallic portion, and t comprises the debinding operation time.

In order to prevent cracking and stress build up during thermaldebinding operations it is necessary to slowly heat the green parts in afurnace. As those skilled in the art will appreciate, such a lengthyprocessing time limits furnace availability and decreases the overallproduction rate of sintered PIM metal parts.

Applicant has discovered, however, that immersion of green parts formedusing Applicant's two phase binder portion in a solvent whichselectively extracts one of the binder components from the molded greenparts forms porosity and channels within that green part. Applicant hasfurther discovered that the resulting porosity/channel structuresignificantly reduces the time required for thermally debinding themolded green part. Furthermore, such solvent extraction procedures donot require large capital expenditure to obtain expensive processingequipment. For example, green parts formed using Applicant's two phasebinder system can be immersed in a solvent bath. In certain embodiments,Applicant's method includes heating that solvent bath. In certainembodiments, Applicant's method includes using a piezoelectrictransducer to emit ultrasonic energy to enhance selective extraction ofone or more binder components from the molded green part.

In certain embodiments, Applicant's two phase binder portion includes ahydrocarbon wax. Applicant has further discovered that use of such a waxwith caprolactam reduces the sensitivity of the molded parts to ambienthumidity. Caprolactam is hygroscopic. Green parts molded from a bindercomprising high levels of caprolactam might soften or swell if exposedto a humid environment over an extended period of time. Applicant hasfound, however, that addition of a wax component to the binder portionimparts hydrophobicity to the binder and reduces green part humiditysensitivity.

Molten caprolactam is miscible with most organic compounds exceptstraight chain aliphatic hydrocarbons. Molten paraffin and polyethylenewaxes, however, are not soluble in molten caprolactam. Applicant's twophase binder system takes advantage of this phenomena wherebycaprolactam forms an emulsion with molten wax. Solidification of thismixture yields a two phase organic system composed of pure caprolactamand wax regions.

In certain embodiments, Applicant's two phase binder system comprises ahigh concentration of extractable caprolactam phase. In certainembodiments, Applicant's binder system comprises more than about fiftyvolume percent caprolactam. In these embodiments, selective removal ofthe caprolactam portion of a molded green part forms an interconnectedpore network through which solvent may percolate.

In order to stabilize Applicant's emulsion binder system, certainembodiments include one or more compatibilizers. Those one or morecompatibilizers are added to the molten binder system. These one or morecompatibilizers stabilize the individual droplet size of the caprolactamdispersed phase, preventing those individual droplets from coalescing.Coalescence is undesirable since it adversely effects binder viscosityand lowers the uniformity of the Ti metal powder dispersed throughoutthe molded green part.

Carboxylic acids have been shown to strongly hydrogen bond tocaprolactam. Useful emulsion compatibilizers will exhibit affinity forboth the nonpolar wax as well as the caprolactam phase. Applicant hasfound that suitable compatibilizers partition at the wax/caprolactaminterfaces comprising the emulsion. Fatty carboxylic acids/amides, suchas stearic acid, or carboxylic acid functionalized waxes such as BakerPetrolite (Sugar Land, Tex.) Unicid 350, are useful compatibilizers. Inaddition, Applicant has found that polyethylene copolymers havingacrylic or methacrylic acid functionality are useful stabilizers for histwo phase binder system. In certain embodiments, Applicant's compositionincludes Primacor 58801 Polyethylene-co-acrylic acid copolymer sold byDow Plastics, Midland, Mich.

In certain embodiments, ethylene acrylic acid copolymers are preferredover fatty acids and polar waxes as compatibilizers due to the abilityof the former to also enhance the toughness of the green molded part. Incertain embodiments, Applicant's compatibilizers include hydrocarbonsfunctionalized with polar head groups such as amide or alcohol moietiescapable of hydrogen bonding with caprolactam. Certain embodiments ofApplicant's two phase binder system include stearamide and/or stearylalcohol. Example solvent extractable binder compositions are detailedbelow.

In certain embodiments, Applicant's composition includes one or morewaxes having a higher melting point than caprolactam. In certainembodiments, the wax component of Applicant's composition has a meltingpoint greater than about 69° C. Such binder systems facilitate removalof the caprolactam from the molded green part by liquefaction/meltingwithout compromising the dimensional integrity of that molded part. Inthese embodiments, the higher melting point wax portion comprises athree-dimensional network maintaining the molded part's shape while themolten caprolactam is solvent extracted from that molded green part.Molten caprolactam has a low melt viscosity and dissolves more rapidlyin solvent than solid caprolactam.

In certain embodiments, the wax portion has a melting point greater thanabout 80° C. to minimize wax recrystallization. Certain embodiments ofApplicant's composition include paraffin, polyethylene, and/orpolypropylene type waxes. Applicant has further discovered that a narrowmolecular weight distribution for the wax component gives a morecrystalline material. A more crystalline wax is generally less solublein the solvent used to selectively remove the caprolactam portion ofApplicant's binder. In certain embodiments, the polydispersity, i.e. theMw/Mn, of Applicant's wax component is about 2 or less, where Mwrepresents the weight average molecular weight and Mn represents thenumber average molecular weight. In certain embodiments, thepolydispersity of Applicant's wax component is about 1.1 or less Incertain embodiments of Applicant's method, in addition to removal viasolvent extractive methods, the molten caprolactam can also be removedusing other binder removal techniques known in the art including viacapillary action by placing the molded green parts in a heated bedfilled with finely ground inorganic particulates such as submicron sizedalumina.

The following Example IV is presented to further illustrate to personsskilled in the art how to make and use the invention and to identifypresently preferred embodiments thereof. This Example IV is notintended, however, as a limitation upon the scope of the invention,which is defined only by the appended claims.

Example IV

The formulation of this Example IV includes a metal portion and a binderportion. Table VII recites the components comprising the binder portion.

TABLE VII Weight Percent Component Of Non-Metal Portion Caprolactam 51.6Polywax 500* Polyethylene Wax 35.6 Primacor 5990I**Polyethylene-co-acrylic acid 12.8 *Baker Petrolite Corp. Sugarland, TX**Dow Plastics Midland, MI

A 1 cm thick rectangular sample of the above composition was prepared byfirst melt blending Primacor with wax at 95° C. followed by caprolactamaddition. The resultant mixture was solidified at room temperatureproducing a white, translucent mass having a high uniformity ofdispersed caprolactam phase. The sample was then immersed overnight in asolvent bath composed of reagent grade isopropanol heated at 70° C.After immersion, the sample was dried and re-weighed revealing a samplemass loss of 49.1 wt. %.

Table VIII recites an embodiment of Applicant's composition whichcomprises the binder system of Table VII.

TABLE VII Component Concentration (wt. %) Titanium powder 81.1Caprolactam 9.8 Polywax 500 Polyethylene Wax 6.7 Primacor 5990I**Polyethylene-co-acrylic 2.4 acidAfter selective removal of the caprolactam portion from a green partformed using the composition of Table VI, that extracted green part hadabout a 48.7 volume percent solid content.

Applicant's invention includes a method to prepare a green body usingApplicant's molding composition. Referring now to FIG. 1, in step 120one more nitrogen-containing compounds comprising between about 3 andabout 20 carbon atoms are selected. In step 130, any additives to beused are combined. In certain embodiments, such one or more additivesare dry blended using conventional mixing techniques. In alternativeembodiments, such one or more additives are melt blended usingconventional techniques. In certain embodiments, the additives of step130 include one or more polymeric materials, one or more waxes, one ormore stabilizers, and/or combinations thereof. The nitrogen-containingcompounds are heated at a temperature and for a time sufficient totransition from a solid phase to a liquid phase. Thereafter, in step 150the additives of step 130 are added to the molten one morenitrogen-containing compounds to form Applicant's binder component. Incertain embodiments, the one or more additives are added as a solidmixture to the liquefied one more nitrogen-containing compounds of step140. In other embodiments, the one or more additives are added as aliquid to the liquefied one more nitrogen-containing compounds.

In step 160, the metal/metal alloy/ceramic/ferrite powder is added tothe liquefied binder of step 150 using conventional mixing technique toform Applicant's molding composition. In step 170, Applicant's moldingcomposition is introduced into a mold. In certain embodiments,Applicant's molding composition is poured into a rubber latex mold. Inalternative embodiments, an automated molding apparatus is used tointroduce Applicant's molding composition into a suitable mold.

In step 180, the mold and its contents are allowed to cool to atemperature sufficient to solidify the molded green body formed usingApplicant's molding composition. In step 190, the solidified green bodyis removed from the mold. In certain embodiments, Applicant's methodtransitions from step 190 to step 205. In other embodiments, Applicant'smethod transitions from step 190 to step 230.

In the embodiments which include step 205, Applicant's methodselectively removes the one or more nitrogen-containing compounds ofstep 120 (FIG. 1) prior to removing the additive compounds of step 130(FIG. 1). In step 210, the molded green body is removeably placed into asolvent bath. The solvent bath includes a solvent which selectivelyremoves, i.e. dissolves, the one or more nitrogen-containing compoundswhile not removing the other binder components, such as the wax portionfor example.

Table IX recites certain Henry's Law parameters and certainOctanol/Water Partition Coefficient values for a variety of solvents.

TABLE IX Octanol - Water Henry's Law Constant k°_(H) PartitionCoefficient Solvent (mol/Kg*bar)* log P_(OW)** Hexane 0.001 4Trichloroethylene 0.11 2.42 Methylene Chloride 0.41 Ethylene Dichloride0.80 Carbon Tetrachloride 0.03 2.64 Benzene 0.18 2.13 Toluene 0.15 2.69Chloroform 0.25 1.97 Diethyl Ether 0.78-1.2 0.77 Ethyl Acetate 8.9 0.66Acetone 30 −0.24 MEK 20 0.28 THF 14-22 0.45 Isopropanol 130 −0.16Ethanol 200 −0.31 Methanol 200 −0.82 Dioxane 140 −0.42*Henry's Constant Values above are for solute solubility in water at298.15 K as reported by the National Institute of Standards andTechnology (“NIST”), at the NIST Chemistry WebBook and NIST Standardreference Database Number 69, July 2001 Release**Octanol—Water Partition Coefficient Values obtained from Handbook ofEnvironmental Data on Organic Compounds—4^(th) Edition, KarelVerschueren, John Wiley & Sons New York, 2001

As those skilled in the art will appreciate, Henry's Law Constant isobtained using the equation:k ^(o) _(H) =P _(solute) /C _(solute) for solute solubility in water at298.15 Kwhere P_(solute) comprises the Partial Pressure of Solute in watersolution, and C_(solute) comprises the Molar concentration of Solutedissolved in water.

In certain embodiments of Applicant's method, the one or more solventsof step 210 have log P_(OW) values less than about 1. In certainembodiments, the one or more solvents of step 210 have log P_(OW) valuesgreater than about −0.9 and less than about 1. As those skilled in theart will appreciate, caprolactam has a log P_(OW) value of about 0.12.

In certain embodiments, Applicant's method transitions from step 210 tostep 230. In certain embodiments, Applicant's method transitions fromstep 210 to step 215 wherein the one or more solvents of step 210 areheated. In certain embodiments, step 215 includes heating the one ormore solvents of step 210 to a temperature greater than about 60° C. Incertain embodiments, Applicant's method transitions from step 215 tostep 230.

In certain embodiments, Applicant's method transitions from step 215 tostep 220 wherein the molded green body/solvent combination is treatedwith ultrasonic sound waves to assist removal of the one or morenitrogen-containing compounds from the green part. Applicant's methodtransitions from step 220 to step 230.

In step 230, the molded green body of step 190 (FIG. 1), or step 210, orstep 215, or step 220, is placed in a suitably dimensioned vacuum ovenwhich is then evacuated until an internal pressure of about 50 mm Hg orlower is reached. In step 240, the internal temperature of the oven isincreased from room temperature, i.e. about 21° C., to about 70° C. overa period of about 30 minutes. Thereafter in step 250, the internaltemperature of the oven is increased from about 70° C. to about 85° C.over a period of about 60 minutes.

In step 260, the oven temperature is maintained at about 85° C. forabout ten hours. Thereafter in step 270, the internal temperature of theoven is increased from about 85° C. to about 250° C. over a period ofabout 120 minutes. Thereafter in step 280, the internal temperature ofthe oven is increased from about 250° C. to about 280° C. over a periodof about 60 minutes. In step 290, the oven temperature is maintained atabout 280° C. for about six hours.

After processing the green body formed using the steps/embodiments ofFIGS. 1 and 2, most of the carbon-based components of Applicant'smolding composition have been removed from the shaped article.Thereafter, the shaped article is “sintered” in a vacuum furnace atreduced pressure to fuse the individual metal particles. Table X recitesthe melting points for the various metals comprising embodiments ofApplicant's molding composition. Green parts formed with themetals/compounds recited in Table X are sintered to produce shapedarticles at temperatures between the recited “Onset SinteringTemperature” and the “Melting Point.”

TABLE X Onset Sintering Compound Melting Point (° C.) Temperature (° C.)WC 2780 1834 W 3410 2250 Ta 2996 1977 Ti 1675 1106 Zr 1850 1221 Re 31802200

In certain embodiments, the shaped article of step 290 is allowed tocool to room temperature, and that article is sintered at a later time.In other embodiments, the sintering process follows the debinding stepsdirectly.

Referring now to FIG. 3, steps 310 through 370 summarize the sinteringof a titanium shaped article. In certain embodiments, step 310 includesheating the green part from the temperature of about 280° C. of step 290to a temperature of about 315.6° C. In other embodiments, in step 310the processed but unsintered green part is disposed in a vacuum oven andthe internal temperature of the furnace is increased to about 315.6° C.over a period of about 60 minutes. In step 320, the furnace temperatureis maintained at about 315.6° C. for about three hours.

Thereafter, in step 330 the internal temperature of the furnace isincreased from about 315.6° C. to about 482° C. over a period of about120 minutes. In step 340, the oven temperature is maintained at 482° C.for 60 minutes. Thereafter, in step 350 the internal temperature of thefurnace is increased from about 482° C. to about 1329° C. over a periodof about five hours. In step 360, the furnace temperature is maintainedat about 1329° C. for about 60 minutes. In step 370, the furnace isallowed to cool to room temperature after which the newly-formed, highdensity, metal/metal alloy part is removed from the furnace.

Use of Applicants' composition and method yields shaped articles havingunexpectedly low levels of oxygen and residual carbon. As those skilledin the art will appreciate, the higher the residual carbon level thelower the mechanical properties of the formed part. For example, oxygenis highly soluble in the titanium metal crystal lattice, and thepresence of such oxygen generally reduces the ductility of the formedpart.

Applicant's formulation described in Example III was prepared usingTitanium powder having an initial oxygen content of about 0.53%. Afterdebinding and sintering using Applicants' method, the sintered Titaniumbody had an oxygen content of about 0.65%. As those skilled in the artwill appreciate, use of Applicants' composition and method yielded asintered Titanium body having an incremental oxygen level of only about0.12%. Moreover, the residual Carbon impurity level in this sinteredTitanium body was less than about 0.088%.

The following Examples V and VI are presented to further illustrate topersons skilled in the art how to make and use the invention and toidentify presently preferred embodiments thereof. These Examples V andVI are not intended, however, as limitations upon the scope of theinvention, which is defined only by the appended claims.

Example V

Component Concentration (Wt. %) Ti-6Al-4V Powder 87.1 ε-Caprolactam 12.2AC 5120 Ethylene-co-Acrylic Acid Wax 0.57 Primacor 5990I 0.13Polyethylene-co-Acrylic Acid

Example VI

Component Concentration (Wt. %) Ti-6Al-4V Powder 83.5 ε-Caprolactam 15.6AC 5120 Ethylene-co-Acrylic Acid Wax 0.80 Primacor 5990I 0.10Polyethylene-co-Acrylic Acid

In EXAMPLES IV and V, 200 mesh Ti-6Al-4V alloy powder having 0.165%initial oxygen assay was compounded with the components recited. Partswere molded, thermally debound, and sintered using Applicant's methoddescribed above. The oxygen assay for the formed parts of EXAMPLES IVand V were about 0.186% and about 0.193%, respectively. As those skilledin the art will appreciate, titanium parts having such minimal oxygenlevels are suitable for use as structural aerospace materials. Moreover,such parts conform to both ASTM Standard B265/Grade 5, and ASM Standard4906.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andadaptations to those embodiments may occur to one skilled in the artwithout departing from the scope of the present invention as set forthin the following claims.

1. A-powder injection molding composition, comprising: a plurality ofparticles selected from the group consisting of a metal powder, a metalhydride powder, a ceramic powder, a ferrite powder, and mixturesthereof; and a binder consisting essentially of caprolactam and apolymeric material selected from the group consisting ofpolyethylene-co-acrylic acid and polystyrene-co-α-methylstyrene, whereinsaid-caprolactam is present at a level of 15 weight percent to 25 weightpercent of said powder injection molding composition.
 2. The compositionof claim 1, wherein said plurality of particles is selected from thegroup consisting of titanium powder, titanium hydride powder, zirconiumpowder, rhenium powder, tantalum powder, tungsten carbide powder, andmixtures thereof.